In conventional video systems, there are two primary requirements for generating a video signal. The first requirement requires a scene to be sampled by a video camera on a periodic basis at a frequency high enough so that when video (comprising the scene) is displayed back to the user, it is perceived as continuous video. Therefore, the samples should be close enough in time and consistent enough such that a user perceives the video as a fluid motion.
The second requirement is that an exposure of an image sensor in the video camera be such that an amount of light impinging on the sensor is within a dynamic range of the sensor and that a level of the exposure is consistent over short periods of time. As such, the light level from frame to frame should be constant so that the video level is constant and does not exhibit a “flicker” caused by exposure variations of the image sensor. The exposure level of the image sensor is controlled either by controlling the amount of light impinging on the image sensor by a mechanical iris or shutter control or by controlling the integration time for the photosites by electronic signals controlling the image sensor (electronic iris control). The electronic iris control is the most economical method and the one preferred in lower cost systems where the cost of mechanical iris systems are deemed too expensive.
The electronic iris mode of controlling the sensor's integration time by electronic signals to the image sensor usually depends on a time-base of the video camera to set the time period for the exposure control. Therefore, the integration time for the image sensor is based on, and a percentage of, the frame or field time of the video camera. This is utilized because the video camera is usually the master time-base of the system where the display is slaved to the time-base set by the video camera. In such a mode, the time-base is constant. There are applications which require the video camera to be a slave device to other time-bases but in these systems, the video camera depends on the master time-base to be sufficiently constant to enable the video camera to properly perform the electronic iris function. In these systems, the camera time-base is usually slaved to the master by use of circuits like Phase Lock Loops (PLL) where the master clock of the video camera is slowly adjusted so that the slave and the master system have the same line or frame period. This technique also depends on the master time-base to be sufficiently constant so that the PLL will stay locked to the master.
Therefore, it is desirable for the present invention to overcome the limitations of conventional video systems that require a consistent time-base for exposure control to generate a video signal and furthermore, enabling independence between the scene sampling rate and the electronic exposure control system. It is also desirable to have the video camera be a slave device to another dynamic master time-base.